Let's break down the passage in its context (DRB throughout; bold mine)—without importing theology into the text, letting it speak for itself.
The author of Hebrews, as the other NT books, very clearly and unequivocally teaches the need to actively persevere to the end to be saved (to be distinguished from persevering to earn salvation), just as this was necessary for the saints of the Old Covenant (citing examples whose relevance depends on said understanding). This is why it comes in warning and exhortation form, and not descriptive form, as some would perhaps prefer.
Beginning at Chapter 3, the author exhorts and reminds the believers of this duty of Christians, who includes himself (v. 6 "we") as no less under the same obligation (Rom 8:12-13):
3:5-6 And Moses indeed was faithful in all his house as a servant, for a testimony of those things which were to be said: 6 But Christ as the Son in his own house: which house are we, if we hold fast the confidence and glory of hope unto the end.
This is a classic conditional (Rom 8:17; Lk 14:27). Here, holding fast the confidence you have at the beginning of Christian life is not optional, but rather being of the household of the Lord is here predicated on such perseverance. In other words, more simply put, 'If we hold fast our confidence unto the end, we are the house of the Lord: we are not the house of the Lord irregardless of our attitude to Christian living and conduct.'
3:7-19 Wherefore, as the Holy Ghost saith: To day if you shall hear his voice, 8 Harden not your hearts, as in the provocation; in the day of temptation in the desert, 9 Where your fathers tempted me, proved and saw my works, 10 Forty years: for which cause I was offended with this generation, and I said: They always err in heart. And they have not known my ways, 11 As I have sworn in my wrath: If they shall enter into my rest. 12 Take heed, brethren, lest perhaps there be in any of you an evil heart of unbelief, to depart from the living God. 13 But exhort one another every day, whilst it is called to day, that none of you be hardened through the deceitfulness of sin. 14 For we are made partakers of Christ: yet so, if we hold the beginning of his substance firm unto the end. 15 While it is said, To day if you shall hear his voice, harden not your hearts, as in that provocation. 16 For some who heard did provoke: but not all that came out of Egypt by Moses. 17 And with whom was he offended forty years? Was it not with them that sinned, whose carcasses were overthrown in the desert? 18 And to whom did he swear, that they should not enter into his rest: but to them that were incredulous? 19 And we see that they could not enter in, because of unbelief.
Harden not your hearts (v. 8) means it is in their power to harden their hearts. And we know that hardening their hearts means they will not enter into His rest (v. 18). The author worries that someone will grow cold (Mt 24:12-13) and become incredulous, (Lk 8:13) or harden their hearts. He says so explicitly, to exhort one another so that they do not become hardened in heart, even citing the Scripture and applying an urgency: "whilst it is still day"—while you still have the opportunity (Jn 9:4). He worries they will be seduced and indeed decieved by sin.
How does he continue immediately after saying this?
4:1 Let us fear therefore lest the promise being left of entering into his rest, any of you should be thought to be wanting.
There is something to fall short of here (holding your confidence to the end). Let us fear obviously doesn't mean a group of people shuddering in uncertainty. It obviously means 'let's make sure that doesn't happen to us.'
In concordance with the abovementioned understanding of let us fear, he says a few verses later:
4:11 Let us hasten therefore to enter into that rest; lest any man fall into the same example of unbelief.
And in yet other words:
4:14 Having therefore a great high priest that hath passed into the heavens, Jesus the Son of God: let us hold fast our confession.
Two verses later, he mentions the inevitability of sinning along the way, but not leaving us a sickness without a cure, he relates how we have available to use the grace of God through Him who can sympathize with your infirmity and proclivity to sin even after we are regenerated (Jas 3:2; Rom 7:19), sought at the throne of grace:
4:15-16 For we have not a high priest, who can not have compassion on **our infirmities: but one tempted in all things like as we are, without sin. *Let us go therefore with confidence to the throne of grace: that we may obtain mercy, and find grace in seasonable aid.
We need to find mercy when we falter (1 Jn 1:9; 2 Pet 1:9), otherwise we have left ourselves in a state no better than those who hardened their hearts and didn't recieve mercy, but remained in their sin.
He briefly remarks about their being slow to understand even the basics of the Faith, and begins Chapter 6 thus:
6:1-3 Wherefore leaving the word of the beginning of Christ, let us go on to things more perfect, not laying again the foundation of [repentance] from dead works, and of faith towards God, 2 Of the doctrine of baptisms, and imposition of hands, and of the resurrection of the dead, and of eternal judgment. 3 And this will we do, if God permit.
The principle things of Christ are identified as such things as repentance, faith, the baptismal ritual, "the laying on of hands of the presbyterhood," (1 Tim 4:14; 2 Tim 1:6), resurrection and judgement, and so foth. The audience of the Epistle were dull of hearing even to these fundamentals. Relevant here is the fundamental nature of repentance from the sins of the old man.
We then get to our passage:
For it is impossible for those who were once illuminated, have tasted also the heavenly gift, and were made partakers of the Holy Ghost, 5 Have moreover tasted the good word of God, and the powers of the world to come, 6 And are fallen away: to be renewed again to [repentance], crucifying again to themselves the Son of God, and making him a mockery. 7 For the earth that drinketh in the rain which cometh often upon it, and bringeth forth herbs meet for them by whom it is tilled, receiveth blessing from God. 8 But that which bringeth forth thorns and briers, is reprobate, and very near unto a curse, whose end is to be burnt.
Breaking this down syntactically will greatly help us:
- "It is impossible ... to be renewed again to repentance."
Why is it impossible to be renewed again to repentance after having been:
- "illuminated," having "tasted also the heavenly gift," and having been "made partakers of the Holy Ghost," have "tasted the good word of God, and the powers of the world to come," and have then "fallen away"?
Because by so doing they are:
- "crucifying again to themselves the Son of God," and "making him a mockery."
He clarifies this in Chapter 10:
10:19-31 Having therefore, brethren, a confidence in the entering into the holies by the blood of Christ; 20 A new and living way which he hath dedicated for us through the veil, that is to say, his flesh, 21 And a high priest over the house of God: 22 Let us draw near with a true heart in fulness of faith, having our hearts sprinkled from an evil conscience, and our bodies washed with clean water. 23 Let us hold fast the confession of our hope without wavering (for he is faithful that hath promised), 24 And let us consider one another, to provoke unto charity and to good works: 25 Not forsaking our assembly, as some are accustomed; but comforting one another, and so much the more as you see the day approaching. 26 For if we sin wilfully after having the knowledge of the truth, there is now left no sacrifice for sins, 27 But a certain dreadful expectation of judgment, and the rage of a fire which shall consume the adversaries. 28 A man making void the law of Moses, dieth without any mercy under two or three witnesses: 29 How much more, do you think he deserveth worse punishments, who hath trodden under foot the Son of God, and hath esteemed the blood of the testament unclean, by which he was sanctified, and hath offered an affront to the Spirit of grace? 30 For we know him that hath said: Vengeance belongeth to me, and I will repay. And again: The Lord shall judge his people. 31 It is a fearful thing to fall into the hands of the living God.
What's worse than dying without mercy by stoning? Eternal death and having your portion with the adversaries of God (Lk 12:46). Why the worse punishment? Because the infinite dignity of Christ is offended, the Spirit of grace who enables us to go to heaven spurned. This is a form of the 'unforgivable' sin—to reject your means of heaven (the Holy Ghost). It's unforgivable not because of the limitations of God's mercy, but because of the nature of forgiveness: God can't forgive someone who does not want forgiven. To want forgiven is completely incompatible with a habitual or life-long, enduring "willful" sinning—to will the sin is to will not-God, and therefore not heaven, and there will be no unwilling residents held hostage in heaven.
The author goes to pains to mention that, while we can falter, for which reason he exhorts us not to, but to hold fast, God has still remained faithful (Jn 6:37):
2 Timothy 2:11-13 A faithful saying: for if we be dead with him, we shall live also with him. 12 If we suffer, we shall also reign with him. If we deny him, he will also deny us. 13 If we believe not, he continueth faithful, he can not deny himself.
That is, it's not that God has failed us, but we have "been found wanting" (Heb 4:1) by God.
Let's hone in on the analogy the author gives to understand what he writes before it:
Hebrews 6:4-6 For it is impossible for those who were once illuminated, have tasted also the heavenly gift, and were made partakers of the Holy Ghost, 5 Have moreover tasted the good word of God, and the powers of the world to come, 6 And are fallen away: to be renewed again to [repentance], crucifying again to themselves the Son of God, and making him a mockery. 7 For the earth that drinketh in the rain which cometh often upon it, and bringeth forth herbs meet for them by whom it is tilled, receiveth blessing from God. 8 But that which bringeth forth thorns and briers, is reprobate, and very near unto a curse, whose end is to be burnt.
(Notice that the person in question is near to a curse, He is not accursed, He has been made a partaker of the Holy Ghost, anything but a reprobate.)
This is the same teaching as Jesus' on His being the Source of Christian virtue and good works, and that unfruitful branches, if they do not "remain" in Him, are cut off, carried away and burned, a clear reference to Hell:
John 15:1-11 I am the true vine; and my Father is the husbandman. 2 Every branch in me, that beareth not fruit, he will take away: and every one that beareth fruit, he will purge it, that it may bring forth more fruit. 3 Now you are clean by reason of the word, which I have spoken to you. 4 Abide in me, and I in you. As the branch cannot bear fruit of itself, unless it abide in the vine, so neither can you, unless you abide in me. 5 I am the vine; you the branches: he that abideth in me, and I in him, the same beareth much fruit: for without me you can do nothing. 6 If any one abide not in me, he shall be cast forth as a branch, and shall wither, and they shall gather him up, and cast him into the fire, and he burneth. 7 If you abide in me, and my words abide in you, you shall ask whatever you will, and it shall be done unto you. 8 In this is my Father glorified; that you bring forth very much fruit, and become my disciples.
9 As the Father hath loved me, I also have loved you. Abide in my love. 10 If you keep my commandments, you shall abide in my love; as I also have kept my Father's commandments, and do abide in his love.
Obedience and bringing forth fruit is not an optional extra and 'its nice if you have it' but 'its not necessary for salvation.' That's not what Jesus teaches or taught, and that's not what Hebrews, as we have clearly demonstrated, taught. He teaches that you abide in Him if, being enabled and empowered by His grace, you "continue in goodness, otherwise you also will be cut off" (Rom 11:22).
So the answer is, yes, it is about falling away from the Faith like those who hardened their hearts of old, and did not enter into the rest of the Lord. To find some underlying 'no, you can't lose your salvation' is just reading into the text which contains quite clear warnings against falling away—not against merely not coming to believe in Christ in the first place.